Currently, many DWDM optical fiber transmission systems operate at 10 Gb/s channel rates. DWDM optical fiber transmission systems operating at channel rates of 40 Gb/s and higher are highly desirable because they potentially have greater optical fiber capacity and also have lower cost per transmitted bit compared to 10 Gb/s and lower channel rate transmission systems. The need for high bit-rate data transmissions of 40 Gb/s or more through optical fibers presents significant technical challenges to the fiber-optic telecommunications industry because such high bit-rate systems are highly susceptible to optical fiber nonlinearities.
The modulation format of 40 Gb/s DWDM transmission systems must be chosen to have high Optical Signal-to-Noise Ratio (OSNR) sensitivity. High OSNR sensitivity means that a low OSNR signal is sufficient to maintain a desired bit error rate (BER) of the transmission or, equivalently, that the system is able to operate at a desired BER even in the presence of a high level of optical noise. In addition, modulation formats of 40 Gb/s DWDM transmission systems must be chosen to be tolerant to optical filtering because existing systems sometimes include optical multiplexers and demultiplexers for 50 GHz channels spacing that limit the bandwidth. Also, modulation formats of 40 Gb/s DWDM transmission systems must be chosen to be tolerant to cascaded optical add-drop multiplexers.
The Differential-Phase Shift Keying (DPSK) modulation format, which is sometimes referred to as Differential Binary Phased Shift Keying (DBPSK), has been considered for use in 40 Gb/s DWDM transmission systems. Using the DPSK modulation format has numerous advantages over the more conventional On-Off Keying (OOK) modulation format that is deployed in many relatively low data rate transmission systems. Differential-Phase Shift Keying transmission systems have been shown to have an approximately 3 dB improvement of OSNR sensitivity compared to OOK transmission systems. However, the DPSK modulation format is more complicated to transmit and receive than the more conventional OOK modulation format.
The Differential Quadrature Phased Shift Keying (DQPSK) modulation format has also been considered for 40 Gb/s DWDM transmission systems. The DQPSK modulation format uses a symbol rate that is one half of the data rate. For example, a 43 Gb/s data rate in a DQPSK system corresponds to a data rate of 21.5 Giga symbols per second. Consequently, DQPSK transmission systems have a narrower spectral bandwidth, greater chromatic dispersion tolerance, and greater tolerance with respect to polarization mode dispersion (PMD) compared with OOK and DPSK modulation formats. However, DQPSK transmission systems have worse receiver sensitivity than DPSK transmission systems. Also DQPSK transmitters and receivers are significantly more complex than OOK and DPSK transmitters and receivers.
Both DPSK and DQPSK modulation formats are used in a non-return-to-zero (NRZ) format where the light intensity is constant between two neighboring symbols and a return-to-zero (RZ) format where the light intensity always drop or return to zero between each symbol. The light intensity returns to zero even if the data signal includes numerous consecutive zeros or ones. Transmitters using RZ-type modulation formats can achieve better OSNR receiver sensitivity and tolerance to fiber nonlinearities than transmitters using NRZ-type modulation formats.
In DPSK and DQPSK transmission systems, the digital information is written in the optical phase of the signal and, therefore, the digital information cannot be detected by ordinary intensity detectors. Differential-Phase Shift Keying receivers use optical demodulators to convert the phase modulated signal to an amplitude modulated signal that can be detected by ordinary intensity detectors.
Both DPSK and DQPSK receivers use one or more optical demodulators that convert the phase modulation of the transmitted optical signal into amplitude modulated signals that can be detected with direct detection receivers. Typically, optical demodulators are implemented as delay interferometers that split the optical signal into two parts, delay one part relative to the other part of the optical signal by a differential delay Δt, and then recombine the two parts of the optical signal to achieve constructive or destructive interference depending on the phase which is modulated onto the optical signal by the transmitter.